Respiratory syncytial virus prophylaxis with nirsevimab: impact beyond hospitalisations

After decades of unsuccessful efforts to prevent respiratory syncytial virus (RSV) infection, the development of new vaccines and monoclonal antibodies has brought renewed optimism. As these tools are implemented across health systems with differing policies, careful evaluations are essential to ensure effective and equitable prevention. In this context, the study by Perramon-Malavez et al.,¹ investigates how a population-based immunisation programme using nirsevimab for infants impacts emergency department (ED) visits and hospitalisations for bronchiolitis in Catalonia (Spain). They compared the results with a similar cohort in the United Kingdom and Italy, where this strategy had not been implemented.

In October 2022, nirsevimab was approved for use in infants under one year of age to prevent RSV-related lower respiratory tract infections. Spain was among the first countries to adopt a universal seasonal immunisation strategy for all infants under 6 months, using a dual approach: administration at birth before discharge from maternity wards, and catch-up immunisation of infants turning 6 months during the RSV season (October–March) via hospitals and primary care centres.² National coverage exceeded 90%. Consistent with pivotal trial results, real-world data from Spain and other countries showed that nirsevimab reduced RSV-related hospitalisations and ICU admissions by over 80%.^3,4

A key remaining question is whether nirsevimab reduces not only hospitalisations, but also ED visits for all respiratory illnesses, especially bronchiolitis. Perramon-Malavez et al. report a clear reduction in ED consultations and hospitalisations for bronchiolitis especially in infants under 6 months, an effect not observed in countries where nirsevimab was not implemented. This study has some limitations including its retrospective design, single season focus, and absence of virological confirmation, assuming RSV as the main cause of bronchiolitis. Nevertheless, their findings align with other studies conducted in ED departments across Spain that confirm reductions in both emergency visits and hospitalisations for bronchiolitis and RSV infections.⁵ Furthermore, this multiregional study supports the greater effectiveness of the dual immunisation strategy over newborn-only approaches adopted in a single region in Spain.

Another interesting finding is the decrease in care for all respiratory causes, including reduced care for non-RSV respiratory infections. While the Perramon-Malavez study could not confirm this due to its design, the reduction was already observed in the pivotal trials and is supported by a multicentre study performed in Madrid,⁶ where a detailed virological analysis across three RSV seasons—including the 2023–2024 nirsevimab campaign—revealed a 62.5% decline in overall respiratory infection-related hospitalisations in infants under 12 months. Specifically, RSV-related admissions dropped by 78%, while reductions were also observed for human metapneumovirus (36.6%) and adenovirus (69.5%). The mechanisms behind these broader effects remain unclear.

The study of Perramon-Malavez et al. represents a unique, perhaps unrepeatable, opportunity to evaluate nirsevimab before the introduction of maternal RSV vaccines, which have been selected as preventive measure in the UK since September 2024 and recommended for all pregnant women year-round. Although initial maternal coverage remains below 50%, the epidemiological context has changed. Preliminary results from Argentina's BERNI study, with 67% maternal vaccine coverage with a seasonal programme, showed 76% efficacy in preventing RSV associated severe cases and hospitalisations.⁷

Data from Spain's second season of nirsevimab implementation, along with ongoing international studies, will be crucial in guiding health policy decisions. Countries must assess not only clinical outcomes but also cost-effectiveness, logistical feasibility, and public health impact. Several unresolved questions remain, including the potential emergence of nirsevimab resistance (which has not been observed to date⁸), and whether infants who received nirsevimab in their first-year experience increased RSV severity in subsequent seasons.

Continued surveillance and comparative analysis of different immunisation strategies—nirsevimab, maternal vaccination, or combined approaches—will be essential for optimising RSV prevention globally. Evidence generated during these critical early implementation phases will shape long-term policies and ultimately help protect infants from RSV globally.

Contributors

Both authors participated equally in the writing of the manuscript and its final approval.

Declaration of interests

The authors declare no conflicts of interest.